ATLAS is one of the two general‐purpose detectors at the Large Hadron Collider (LHC). The trigger system needs to efficiently reject a huge rate of background events and still select potentially interesting ones with good efficiency. After a first processing level using custom electronics, the trigger event selection is made by the High Level Trigger (HLT) system, implemented in software. To reduce the processing time to manageable levels, the HLT uses seeded, step‐wise and fast selection algorithms, aiming at the earliest possible rejection of background events. The ATLAS trigger event selection is based on the reconstruction of potentially interesting physical objects like electrons, muons, jets etc. The recent LHC startup and short single‐beam run provided the first test of the trigger system against real data. Following this period, ATLAS continued to collect cosmic‐ray events for detector alignment and calibration purposes. Both running periods provided very important data to commission the trigger reconstruction and selection algorithms. Several tracking, muon‐finding, and calorimetry algorithms, constituting the first ATLAS trigger menu, were exercised online. The trigger decision was used to stream the events into separate samples. This event streaming facilitated the commissioning of the different ATLAS sub‐detectors. This paper will give an overview of the trigger design and its innovative features. It wil l focus on the valuable experience gained in running the trigger reconstruction and event selection in the fast‐changing environment of the detector commissioning

Urquijo, P

ATLAS is a particle physics experiment at the CERN Large Hadron Collider (LHC), designed to discover new physics beyond the Standard Model as well as for precise measurements of known particles and interactions. The first $p-p$ collisions are foreseen later in 2009. The short LHC single-beam run in 2008 provided a ``stress test'' of the trigger. ATLAS has since collected more than $300 imes10^6$ cosmic-ray events for detector alignment and calibration purposes. These running periods allowed the trigger to be tested in different running conditions. This proceedings focuses on the experience gained in running the trigger in various commissioning environments

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ATL-DAQ-PROC-2009-03428October2009Proceedings of the XXIX PHYSICS IN COLLISION 1Commissioning of the ATLAS Trigger Event Selection with Single Beamand Cosmic RaysPhillip Urquijo1 (on behalf of the ATLAS Collaboration)(1) University of Geneva, Geneva, SwitzerlandAbstractATLAS [1] is a particle physics experiment at theCERN Large Hadron Collider (LHC), designed to dis-cover new physics beyond the Standard Model as wellas for precise measurements of known particles and in-teractions. The first p − p collisions are foreseen laterin 2009. The short LHC single-beam run in 2008 pro-vided a “stress test” of the trigger. ATLAS has since col-lected more than 300×106 cosmic-ray events for detectoralignment and calibration purposes. These running pe-riods allowed the trigger to be tested in different runningconditions. This proceedings focuses on the experiencegained in running the trigger in various commissioningenvironments.1. IntroductionThe trigger system needs to efficiently reject a hugerate of background events and still select potentially in-teresting ones with high efficiency. After a first process-ing level using custom electronics, trigger event selectionis made by the High Level Trigger (HLT) system, imple-mented in software. To reduce the processing time tomanageable levels, the HLT uses seeded, stepwise andfast selection algorithms, aiming at the earliest possi-ble rejection of background events. The ATLAS triggerevent selection is based on the reconstruction of poten-tially interesting physical objects, e.g. electrons, muons,jets as well as global event properties such as transversemissing energy EmissT. ATLAS has a trigger scheme withthree distinct levels. Level-1 (L1) takes information fromthe calorimeter and muon detectors and has the task toreduce the input bunch crossing rate to around 75 kHzin a fixed latency of maximum 2.5 µs. The L1 muontrigger (L1Muon) uses track information retrieved fromthe resistive plate chambers (RPC) in the barrel, andthe thin gap chambers (TGC) in the end-cap region,to identify high-pT muon candidates. The L1 calorime-ter trigger (L1Calo) uses calorimeter energy deposits toidentify high ET electrons, γs, τs as well as jets, and cal-culate missing and total energy sums from the calorime-ter. Data are transferred to the Level-2 (L2) CPU corre-sponding to geometrical regions of interest (RoI) withinthe detector identified by L1 as containing possible inter-esting signatures. L2 evaluates the event characteristicslooking only at a portion of the event data and reducesthe event rate to around 3 kHz and the bandwidth toaround 3 GB/s. The L2PUs execute software algorithmsthat request event fragments around the RoI specifiedat L1. If the event is accepted the Event Filter (EF)performs its selection after the event is fully assembled,typically using oﬄine algorithms. Dedicated algorithmswere created for commissioning purposes during the sin-gle beam and cosmic runs.2. LHC single beam runThe main trigger goal for first beam was for reli-able, stable operation, demanding a simple configurationbased on L1 decisions only. It was also crucial for theBeam Pickups (BPTX) and Minimum bias scintillatorsFig. 1. Timing distribution of L1 triggers from September 12,the third day of single-beam data.(MBTS) to be well timed in. During this period it wasessential to ensure that the detectors were not damagedby the unstable conditions, thus the pixel detector wasswitched off, the semi-conductor Tracker (SCT) was setto a low bias voltage, and the muon system was set toa reduced high voltage. Although not used for activerejection, the HLT infrastructure was used for taggingevents and routing them to their streams. Fewer than1000 events were kept in this period, which required thatL1Calo or L1Muon be in time with BPTX or MBTS. Animportant achievement during first LHC operation wasthe first synchronisation of the trigger signals at the levelof one bunch crossing (25 ns). Figure 1. shows the tim-ing distribution of L1 triggers from Sep. 12, the thirdday of single-beam data. Events were triggered with theMBTS. Of note in this plot is the excellent timing of theBPTX and MBTS, and the relatively small coincidenceof the minimum bias trigger with the calorimeter andmuon triggers. The latter implies that the beam qualityhad improved significantly in the first 48 hours of beamtime. The timing of the RPC had not been tuned at allprior to this run.3. Cosmic runsA more complete test of the HLT was possible dur-ing the subsequent cosmic ray runs. In 2008, startingon September 13th, ATLAS collected 216 million eventsuntil the end of October. These runs presented the firstopportunity to fully exercise the whole detector. Cosmicrays present particular challenges to the HLT, which hasbeen designed to perfom best for collision events. Incosmic events tracks are distributed over detector ratherthan from beam interaction point (IP), however triggeralgorithms are optimised to select tracks that originatefrom the IP. Also, cosmic muons rarely provide the kindof signatures that initiate trigger chains associated tonon-muon physics objects in the trigger menu, such ascalorimeter clusters from electrons or jets. This severelylimits the statistics that can be used to test a lot of thetrigger chains prepared the collision run. Despite theseconstraints, an effort was made to exercise all the menuchains during the cosmic ray running. Adaptations tothe tracking algorithms were required for the cosmic trig-c©2009 by Universal Academy Press, Inc.2Fig. 2. Energy measured in trigger towers by the L1 calorimetertrigger (x-axis) compared to the energy measured in the fullreadout for the calorimeters (y-axis).ger chains and un-modified algorithms were exercised ina full collision that menu that ran in parallel with thecosmic chains.An example performance test of the L1Calo trigger inthis period is shown in Fig. 2., where the energy mea-sured in the electromagnetic calorimeter trigger towers(0.1×0.1 in η/φ) by L1Calo (x−axis) is compared to theenergy measured in the full readout for the calorimeters(y−axis), with clear correlation. A sharp cut-off is seenin the trigger tower energy at the trigger threshold of 5GeV used for much of the 2008 cosmic running. For theHLT, the most important aspect of commissioning withcosmics was in data preparation; responsible for provid-ing calorimeter cell information to the HLT trigger algo-rithms. An important performance requirement of theL2 trigger is that the average processing time should be∼40 ms. L2 electron and photon triggers, for example,execute algorithms firstly over calorimeter data. Themean RoI processing time was found to be 3.9 ms. Itwas also demonstrated that the results from L2 and EFare in excellent agreement, meaning that the faster, sim-pler L2 algorithm performs adequately with respect tothe more complex EF quasi oﬄine reconstruction algo-rithm.Since cosmic rays are distributed over the entire vol-ume of the detector, the purpose of the IDCosmic selec-tion was to select a subset of events passing through theinner detector to output to a separate stream used forID alignment. Three parallel selections were used at L2,2 (SiTrack and IDScan) based on track reconstructionstarting in the Pixels and SCT and extended to the TRT,and one (TRTSegmentsFinder) based on the TRT alone.Minor changes were made to the IDScan and SiTrack al-gorithms to adapt them for the cosmic chains so as toensure high efficiency for tracks passing up to ∼100 mmfrom the interaction region. Relaxation of the require-ment for tracks to come from the IP increases the sensi-tivity of the algorithms to detector noise, so additionalprotection against noisy detectors was added. The TRT-SegmentFinder algorithm was purpose built for cosmicchains. Fig. 3. shows L2 event reconstruction efficiencyas a function of transverse impact parameter, d0 for 2008cosmic data. L2 efficiency is defined with respect to anevent with a good oﬄine track. The efficiency is greaterthan 99% with a fake rate of 0.01%-1% (i.e. events witha trigger track but no corresponding oﬄine track).Two parallel cosmic muon chains were exercised, onebased on muon detector information alone and the otherusing calorimeter and inner-detector information. Ex-tensive studies were possible for the stand-alone muon-detector triggers by relaxing the requirement for tracktrajectories come from the IP. The L2 efficiency wasmeasured to be 93% for tracks in the end-caps. Lossesare mainly attributable to detector timing issues specificoffline track d0 [mm]-400 -300 -200 -100 0 100 200 300 400L2 event trigger efficiency0.50.60.70.80.911.1ATLAS preliminaryAny L2 ID trackSiTrackIDScanTRTFig. 3. L2 reconstruction efficiency as a function of track impactparameter d0.m_h_etaResEFOff_csEntries  1313Mean   -0.0007389RMS    0.04435 difference EF - offlineη-0.4 -0.3 -0.2 -0.1 -0 0.1 0.2 0.3 0.4Number of entries110210 eta res ef matched cosmicsATLAS Preliminary2008 cosmics dataEvent FilterMuon SpectrometerFig. 4. Difference in η between muon tracks obtained in theMuon Spectrometer by the EF, and the corresponding muontracks obtained by oﬄine reconstruction.to cosmic running. In addition, an L2 algorithm thatsearches for low pT µs in the Tile Calorimeter (TileCal)was run. To reduce the effect of electronic noise, theenergy measured in the calorimeter was required to beabove 300 MeV. The energy deposition of these parti-cles peaks at approximately 2.5 GeV, consistent withthe energy loss of a minimum ionising particle. Cosmicmuons were identified by the L2 trigger by combining in-formation from the ID and Tile calorimeter algorithms(solenoid and toroid B fields on). The performance ofthe EF muon algorithm was tested by comparing thedifference between the track parameters (φ and η) de-termined by the EF algorithm and the same track pa-rameters reconstructed oﬄine (Fig. 4.). The distribu-tions have non-gaussian tails due to differences in thecalibration constants used online and oﬄine and due todifferences between online RoI-based reconstruction andfull-event reconstruction oﬄine.4. ConclusionThe cosmic and single beam running periods providedvery important data to commission the trigger recon-struction and selection algorithms. The tracking, muon-finding, and calorimetry algorithms were successfully ex-ercised online as part of a full trigger menu. The triggerdecision was used to stream the events into separate sam-ples. This event streaming facilitated the commissioningof the ATLAS sub-detectors.References[1] G. Aad et al., (ATLAS Collab.) JINST 3 (2008)S08003.

Commissioning of the ATLAS Trigger Event Selection with Single Beam and Cosmic Rays

ATL-DAQ-SLIDE-2009-22426August2009Commissioning of the ATLAS Trigger Event Selectionwith Single‐Beam and Cosmic RaysPhillip UrquijoUniversity of Geneva, SwitzerlandOn behalf of the ATLAS collaborationPhysics in Collision, Kobe 2009Phillip Urquijo, University of Geneva, PIC 2009 KobeCommissioning of the ATLAS Trigger Event Selection with Single‐Beam and Cosmic RaysThe trigger must reject a huge rate of background events and select potentially interesting ones with high efficiency. First level using custom electronics (L1).Event selection made by High Level Trigger (HLT), implemented in software. Step‐wise selection and fast reconstruction algorithmsEvent selection based on the reconstruction of potentially interesting physical objects e.g. electrons, muons, jets, taus, and and global event properties such as missing ET.2~ 3 kHz~ 200 HzRead-Out SystemATLAS is one of the two general‐purpose detectors at the Large Hadron Collider (LHC). Phillip Urquijo, University of Geneva, PIC 2009 Kobe 3Since then ATLAS continued to collect over 300 million cosmic‐ray events for detector alignment and calibration purposes. Provided very important data to commission the trigger algorithms. Tracking, muon‐finding, and calorimetry algorithms, constituting the first ATLAS trigger menu, exercised online. Trigger decisions used to stream events into separate samples. Facilitated the commissioning of the different ATLAS sub‐detectors.The 2008 LHC startup and short single‐beam run provided the first test of the trigger system with real data. Commissioning of the ATLAS Trigger Event Selection with Single‐Beam and Cosmic RaysPhillip Urquijo, University of Geneva, PIC 2009 Kobe• Trigger architecture• HLT software structure• First experience with LHC beam• Timing in the triggers• Commissioning with cosmics in 2008/2009.• Calorimeter triggers• Tracking triggers• Muon triggers4The poster focuses on the valuable experience gained in running the trigger reconstruction and event selection in the fast‐changing environment of detector commissioning.OutlineCommissioning of the ATLAS Trigger Event Selection with Single‐Beam and Cosmic RaysCosmic muon traversing the inner detector

Commissioning of the ATLAS Trigger Event Selection with Single‐Beam and Cosmic Rays

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